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Appendix II.

8/5

With friction, the real resistance is R2, and the force diagram
A B D, where 0 = friction angle.

W ^ A B _         i    '

Po         BD

tan (0 + 0)
P          tan 9

P2      tan (0 + 0)

shewing that large pitch is economical.    Returning to the experi-

and efficiency ==

ment,

/n      x       tan 0
tan (0 + 0)  = -_- =
effy. "•

'3535
•. 0 = (0 + 0)~ 0 = 13*27°-476° = 8-51°

and   IJL — tan 0 = "1494       (Seep. 1125.)

Absorption Dynamometer.— The apparatus in
Fig. 596 is called an Appold 'brake. If P = pull on stud D,
r = radius of brake wheel, and Fn total friction on "brake strap,
the sum of moments being necessarily zero,

(W-S)R ± (PxAD) = Fnr

or the total moment exerted by the-.engine. P may be measured
by two spring balances, one on each side of D, a pull on the right
balance being plus, and on the left minus. The work absorbed
per revolution would be          • ' '

2 ^{(w - S)R ± (P x A D)}

whence B.H.P. is found. If the H.P. be under 15 and the
lubrication sparing, there is little pull on r>, but the lever is
generally a bad arrangement, and a simple strap is now advised,
where only W and S are measured.

P. 580. Distribution of Power.^—We may distinguish
between mere transmission, and distribution from a central station
to many consumers. Professor Unwin has shewn the advantages
of the latter over individual installations, and gives the following
requirements:

i. Indefinite subdivision and measurement.

af. Minimum first cost and mntxing- loss.           '                    *

3. Freedom from danger.                                         • • . i

3 -M